cooling source for precooling vegetables -...

Evaluation of stored winter coldness as acooling source for precooling vegetables

C. VIGNEAULT1, J.GALLICHAND2, L. BLOUIN3 and G. JACOB3

Agriculture Canada Research Station, St. Jean-sur-Richelieu, PQ, Canada J3B 6Z8;2P.O.Box 148, Macdonald College, McGillUniversity, Ste.Anne-de Bellevue,PQ, Canada H9X ICO; and Roche et AssocUs, 2535 boul.Laurier, Ste. Foy, PQ, CanadaG1V4M3. Received 11 March 1987; accepted 8 May 1990.

Vigneault, C, Gallichand, J., Blouin, L. and Jacob, G. 1990. Evaluation of stored winter coldness as a cooling source for precoolingvegetables. Can. Agric. Eng. 32:285-289. A system was built to forman ice block during winter and to use it in summer as a cooling sourcefor precooling vegetables. The experiments took place from November1983 to September 1985. Ice was formed by applying at regularintervals a layer of water exposed to a forced air current. The volumeof ice formed was 325 m during the second winter when the systemwas functioning properly. The efficiency and energy requirements ofthe ice block precooling system were compared to a conventionalprecooling system that uses a mechanical refrigeration unit. The use ofthe ice block allowed to maintain an average water temperature of0.7°C compared to 1.3°C for the conventional system. The colderwater obtained with the ice block system reduced the precooling timeand resulted in a vegetable precooling capacity of 3.0 t/h compared to1.3 t/h for the conventional system. Energy requirements were reducedfrom 32.2 kWh/t of vegetables for the conventional system to 5.4kWh/t for the ice block system.

INTRODUCTION

The temperature of vegetables needs to be reduced to 4°C orless as soon as possible after harvest in order to delay productdeterioration (Kader et al. 1985). This rapid removal of heat,called precooling, isachieved bycirculating cold water (0- 2°C)through the vegetables. By reducing the respiration rate of thevegetables, precooling contributes to the maintenance of agood quality and good looking product, which is particularlyimportant for fresh market vegetables such as sweet corn,broccoli, bunched carrots, cauliflower, green onions, radishesand celery.

With conventional precooling systems, water is cooled bythe evaporator of a mechanical refrigeration unit. These systems have a high energy consumption, and many vegetablegrowers do not precool their vegetables because of high operating costs.

Because of its geographical location, Canada has a coldwinter climate. Buies (1983) showed the possibility of storingwinter coldness in the form of an ice block and to use it as a

cooling source for air conditioning of buildings. Vigneault andLevesque (1984) and Vigneault and Lemieux (1985) developed a system for producing an ice block that could be used forprecooling vegetables.

The objective of this study was to evaluate the technicalfeasibility of using an ice block formed during winter as acooling source for precooling vegetables, and to compare theefficiency and energy requirements of this system with a conventional system.

CANADIAN AGRICULTURAL ENGINEERING

MATERIALS AND METHOD

The study was conducted from November 1983 to September1985 in the processing and storage facilities of a vegetablegrowers' cooperative near Quebec City. The existing conventional precooling unit was relocated in a building large enough(18.2 m long, 12.2 m wide, 7.0 m high) to accommodate theice block. This building was previously used as a storage areaandwaswellinsulated(Rvaluesof 4.02m °C/Wfor thewallsand 4.57 m2 °C/W for the ceiling).

Conventional precooling system

Figure 1 presents a schematic representation of the conventional precooling system. It consisted of a water reservoir, aturbine pump, an evaporator coil connected to a mechanicalrefrigeration system, and a conveyor for the vegetables. Inoperation, the water was pumped from the reservoir below theconveyor to another reservoir above the vegetables at a rate of80 L/s. The upper reservoir, containing the evaporator coil,had its bottom perforated to allow a uniform distribution ofwater through the vegetables. The system had a total electricpower requirement of 41.8 kW, which included 7.5 kW for thepump and 0.7 kW for the conveyor. The energy required forremoving the heat from the vegetables was provided by a 33.6kW condensing unit consuming 29.9 kW for the compressorand 3.7 kW for the condenser fan.

rfJTmi

1. WATER

2. PUMP

3. EVAPORATOR

4. TO COMPRESSOR

5. VEGETABLES

Fig. 1. Conventional precooling system for vegetables.

285

Ice block system

The ice block system consisted of an ice forming system, usedto form the block of ice during winter, and an ice blockprecooling system needed to precool the vegetables in summer. The following two sections briefly describe thecomponents and operation of these two systems. A completedescription of the system including construction specificationscan be found in GaUichand and Jacob (1984) and in Blouin andJacob (1985).

Ice forming system. Figure 2 shows the components of theice block system and the location of the ice block with respectto the conventional precooling system. The area used to buildthe ice block consisted of a wooden structure (11m long, 8 mwide, 4.3 m high) made watertight on the first meter by aplastic liner, and insulated from the concrete floor to avoidfreezing damages to the building foundation.

The ice forming system consisted of a water distributionsystem and an air distribution system. These systems wereattached to a mobile aluminum structure, fitting into thewooden structure. The mobile structure was raised manuallyas the ice block height increased. The air distribution systemprovided cold air from the outside to freeze the water appliedby the water distribution system. It consisted of an air intake,a fan, a vertical flexible conduit, a series of horizontal polyethylene tubes perforated at the bottom, and an air outlet. Thewater distribution system was composed of a water reservoirand a centrifugal pump of 1.1 kW, a flexible water supply lineconnected to a pipe network attached to the mobile structure,and a series of 12 micro-sprinklers located below the polyethylene tubing. Ice was produced using the layer by layer iceformation technique developed by Vigneault and Lemieux(1985). This technique consisted of applying at regular time

intervals, layers of water of progressively greater thickness asthe outside temperature decreased. The time interval betweentwo water applications and the depth of water applied wereautomatically controlled by an electric system connected to aseries of thermostats outside the building that were set attemperatures of -8, -14, and -18°C.

Ice blockprecooling system. The existing conventional precooling system was modified so that a series of gates near theprecooler turbine pump intake allowed the operation of thesystem with either the conventional system or the ice blocksystem. The operation and components of the ice block precooling system are shown in Fig. 2. With this system, coldwater was pumped with the turbine pump (9 right, Fig. 2) fromthe basin containing the ice block into the perforated upperreservoir of the precooler at a flow rate of 80 L/s. Aftercirculating through the vegetables, water was pumped fromthe lower reservoir onto the ice block by a 7.5 kW centrifugalpump (9 left, Fig. 2). A valve (10, Fig. 2) was installed downstream of the centrifugal pump to allow for variation of therecirculation flow rate on top of the ice block between 0 and63 L/s. An overflow channel (8, Fig. 2) was built around thewooden structure to circulate, by gravity around the ice block,the water that was not pumped by the centrifugal pump.

Experimental procedure

The process of ice formation was closely monitored during thetwo winter seasons of the study. The objective of this monitoring was to determine optimal values for the parameterscontrolling the rate of ice formation, namely the air flow rate,the time interval between two water applications and thedepths of water applied corresponding to the three set-pointsof the thermostats. For both seasons, the ice formation period

S!©!

Fig. 2. Vegetable precooling system using an ice block.1) air distribution system intake; 2) mobile structure; 3) water supply line; 4) ice melting system; 5) ice block;6) plastic liner used to form a watertight reservoir; 7) air outlet; 8) overflow channel; 9) right: turbine pump;9) left: centrifugal pump; 10) adjustable valve; 11) upper precooler water reservoir; 12) vegetables being precooled;13) lower precooler reservoir.

286 VIGNEAULT, GALLICHAND, BLOUIN and JACOB

extended from early December to late March.Tests of the two precooling systems were conducted for ten

days during the summer of 1983 and for five days during thesummer of 1984. The vegetables tested were sweet corn andbroccoli. Table I presents the distribution of the 30 tests performed, according to the type of precooling system and thetype of vegetable. Whether the conventional system or the iceblock system was used, the precooling was mainly done bybatches: the vegetables were loaded in the precooler, its doorsclosed, and water circulated through the vegetables for a period of time. The duration of the precooling was determined bythe temperature in the center of selected samples, as measuredby thermocouple probes connected to a digital datalogger.The tests were stopped when the temperature at the center ofthe vegetables reached 8°C. This procedure resulted in anaverage temperature of 4°C for the total mass of vegetablesafter precooling. Before and after each test, the vegetableswere weighed and their temperature measured. During thetests, temperatures were measured at one-minute intervals inthe upper and lower reservoirs of the precooler. Water sampleswere taken on 15 occasions during the testing periods andanalyzed for coliforms, suspended matter and dissolved solidsusing standard methods (APHA 1981).

Table I. Distribution of the precooling tests.

Type of precooling system

Vegetables Conventional Ice block

Sweet com

Broccoli

6

3

18

3

RESULTS AND DISCUSSION

Ice formation

The volume of ice formed was 225 m for the 1983-84 season

and 325 m3 forthe1984-85 season. Alowervolume of icewasformed during the first season due to a non-uniform distribution of the air over the ice block. This resulted in a

non-horizontal surface having vertical elevation differences ofup to 1.22 m. The non-uniformity of the air distribution wascaused by an insufficient capacity of the electric motor of thefan (0.56 kW) which provided 6,163 L/s of air. For the secondseason, the electric motor of the fan was changed (1.5 kW) andthe resulting air flow of 8,137 L/s was adequate to ensure auniform distribution of the cold air on top of the ice block.

Some trials and adjustments were necessary to determinethe time interval between water applications and the optimaldepth of water to be applied for each thermostat set-point.Table II presents the operational parameters of the ice formingsystem that were found most satisfactory during the 1984-85season. The efficiency of an ice forming system can be expressed in terms of degree-minutes of freezing per mm ofwater applied (DMF/mm). Buies (1983) defined the DMF/mmunit as the product of the absolute value of the temperaturebelow 0°Cand the timein minutes required to freeze a 1 mmlayer of water. The air flow rate and uniformity of air distribution are the most important factors controlling the efficiencyof the ice forming system. For the 1983-84 season the average


efficiency of the system was 301.9 DMF/mm. This efficiencywas improved to 190.3 DMF/mm for the 1984-85 seasonbecause of the higher air flow rate obtained with the newelectric motor of the fan. The value of 190.3 DMF/mm compares well with the value of 200 DMF/mm obtained by Buies(1983) for a smaller system.

Table IL Operational parameters for the ice formingsystem.

Operational

Parameter Value

Depth of water applied- for a -8°C thresholdtemperature 0.78 mm- for a -14°Cthresholdtemperature 1.42mm- for a -18°Cthresholdtemperature 2.05 mm

Threshold temperature for the fan - 3°C

Time interval between water applications 23 min

(1) Temperature below which the corresponding waterdepth is applied at regular intervals,

(2) Temperature above which the fan does not operate.

Precooling of vegetables

Comparison between the twoprecooling systems. The capacityof the two cooling sources (ice block and mechanical refrigeration unit) to absorb the heat picked up by the water whenpassing through the vegetables was compared by averagingwater temperature readings taken during the tests in the upperreservoir of the precooler. Temperature readings were takenevery minute for the duration of the tests. From the results,summarized in Table III, we see that the average temperaturein the upper reservoir was 0.7°C for the ice block systemcompared to 1.3°C for the conventional system. This difference may be partly attributed to factors other than the coolingsource, such as the initial temperature of the water and themass and initial temperature of the vegetables. However, ascan be seen in Table III, the higher initial water temperatureand higher vegetable temperature for the conventional systemwas counterbalanced by die higher mass of vegetables for theice block system.

Figure 3 illustrates the variation of the water temperature inthe upper reservoir as a function of time for a typical test of theconventional and ice block systems. In the case of the conventional system, the water temperature in the upper reservoir roseat the beginning of the test, thus reducing the temperaturedifference between the water and the vegetables. Heat transferwas therefore slowed down and the cooling time for the vegetables was increased. When the ice block was used, the watertemperature steadily fell from the beginning of the test. Heattransfer between the vegetables and the water was acceleratedand the cooling time was reduced. The colder water obtainedwith the ice block resulted in an average precooling capacityof 3.0 t of vegetables per h compared to 1.3 t/h with theconventional system, which represented an increase of 130 %of the precooling capacity.

Recirculationflow rate on top of the ice block. Water recir-

287

Table HI. Comparison of temperatures obtained withthe two precooling systems.

Table IV. Effect of the rate of water recirculation over the

ice block on the water temperature in the upper

Average

initial

water

temp.

(°C)

Average Averageupper mass of

reservoir vegetables

temp. treated

(°C) (kg/test)

Average

vegetable

initial

temp.

(°C)

1CSC1 run ui we pi CIUU1C1 •

Type of Number of

system tests

Recirculation

flow rate on

the ice block

L/s

Number

of tests

Averageinital water

temperature

<°C)

Average upper

reservoir water2

temperature

(°C)

Ice block 21

Conventional 9

1.0

1.4

0.7 1150

1.3 780

21.5

24.5

63.4

48.8

27.6

7.0

0.0

1

5

8

1

6

0.7

1.1

0.9

0.2

1.4

0.3

0.7

0.5

0.6

(1) Average water temperature in the lower reservoir of the precoolernrior tn the heainninjy nf flip, taste.

0.6

(2) Average water temperaturein the upper reservoirof the precoolerduring the tests.

2.0 t CONVENTIONAL

20 30 40 50 60 70

TIME AFTER PUMP STARTS ( min )

Fig. 3. Water temperature comparison between the iceblock system and the conventional system forprecooling 1 t/h of sweet corn introduced at 20°C.

culation over the surface of the ice block resulted in a releaseof the heat picked up from the vegetables. This heat transferbetween water and ice took place on the top and sides of theice blockwhen the waterwas recirculated by the centrifugalpump, and around the ice block when the water was recirculatedby the overflowchannel.Since the centrifugal pumpwasthecomponent of the ice blocksystem withthe highestenergyrequirement, tests were performed to determine the minimumrecirculation flow rate on top of the ice block that would notincrease the temperature of the water coming out of the icestorage reservoir. The 21 tests performed with the ice blockprecooling system were carried out with recirculation flowrates ranging from 0 to 63 L/s. The mass and initial temperature of the vegetables varied little from test to test so that thewater temperature in the upper reservoir was unlikely to beaffected by thesetwofactors. Theaverage watertemperaturesin the upper reservoir of the precooler are presented in TableIV for each recirculationflow rate. Water temperatures in theupper reservoir were always lower than the initial water temperature, except for the test done with a recirculation flow rateof 7.0 L/s, for which leakage of one of the precooler gatesrequired theaddition of tapwater(at 12°C) during thetest.The

288

(1) Average water temperature in the lower reservoirof the precoolerprior to the beginning of the tests,

(2) Average water temperaturein the upper reservoirof the precoolerduring the tests.

recirculation flow rate seemed to have very little if any effecton the average temperature in the upper reservoir of the precooler. Therefore, adequate precooling can be achieved byrecirculating the total flow around the ice block by the overflow channel, which eliminates the need for the recirculationpump, and reduces the capital investment and energy requirements of the ice block system.

There was always a part of the total recirculation flow ratewhich had to pass by the overflow channel, causing moremelting on the sides and under the ice block. At the end of the1983-84season, this melting caused the ice block to collapseunder its own weight rupturing the plastic liner. Due to theimportance of the leakage the testing program could not becontinued for that year. The plastic liner was repaired duringthe autumn of 1984 to allow continuation of the experimentsin 1984-85. Possiblesolutionsto thisproblemwerenot implemented due to financial limitations.

Water quality. Fifteen water sampleswere takenduringthetests and analyzed to detect possible detrimental effect on thewater quality from using the ice block. Coliforms were notfound in the water. The quantity of suspended matter variedbetween 20 and 59 mg/L. A value of 59 mg/L is high fordrinking water but is not harmful when used for precoolingvegetables. The total dissolved solids ranged between 115 and404 mg/L which is below the recommended maximum valueof 500 mg/L for drinking water (Anonymous 1979). The lowvalues for the total dissolved solids are explainable by themelting of the ice block which continually added new water tothe system.

Energy requirements. Total energy requirements are presented in Table V for each component of the two precoolingsystems. Energy requirements were lower for the ice block(8.2 kW) than for the conventional system (41.8 kW). Moreover, the ice block system precooled vegetables in a shorterperiod of time than the conventional system due to the colderwater obtained with the ice block. Based on average precooling capacities obtained during the tests, the energy required toprecool one tonne of vegetables is 32.2 kWh for the conventional system and 2.7 kWh for the ice block system. However,the latter figure does not take into account the energy requiredby the ice forming system. For the 1984-85 season, when the

VIGNEAULT, GAUICHAND, BLOUIN and JACOB

ice forming system was functioning properly, 1,100 kWh wereconsumed to build the ice block. Not all the ice formed duringthe winter was available for precooling vegetables since someice melted between the end of the ice forming period and thebeginning of the precooling operations. The amount of iceremaining after melting was enough to precool approximately400 t of vegetables. Therefore, ice production represented anenergy requirement of 2.7 kWh/tonne of vegetables. This results in an overall energy requirement of 5.4 kWh/tonne ofvegetables for the ice block system compared to 32.2 kWh/tfor the conventional system, a reduction of 83.6%.

Table V. Energy requirements for the two precoolingsystems.

Energy requirement(kW)

Source Conventional Ice block

Precolling svstem

Turbine pump 7.5 7.5

Conveyor 0.7 0.7

Compressor 29.9

Condenser fan 3.7

Total 41.8 8.2

Ice forming systemWater pump 1.1

Air fan 1.5

Total 2.6

CONCLUSIONS

The overall conclusion of this study is that it is technicallyfeasible to produce a block of ice during winter for use duringsummer to precool vegetables.

The layer by layer ice forming technique used to producethe ice block has proven adequate. The operational parametersof the ice forming system (i.e. air flow rate, depth of waterapplied and time interval between two water applications),


determined by trial and adjustment, allowed the formation of325 m3 ofice with an efficiency of190.3 DMF/mm.

During the precooling tests, the water temperature wascolderwiththeiceblocksystem (0.7°C) thanwiththeconventional system (1.3°C) which uses a mechanical refrigerationunit. The energy required for precooling was decreased from32.2 kWh/tonne of vegetables for the conventional system to5.4 kWh/t for the ice block system.

REFERENCES

ANONYMOUS. 1979. Guidelines for Canadian drinking waterquality. Health and Welfare Canada, Ottawa, ON. 79 pp.

APHA. 1981. Standard methods for the examination of water

and wastewater. 15th ed., American Public Health Association. Washington, D.C., USA. 1134 pp.

BLOUIN, L. and G. JACOB. 1985. Systfeme de pr6-refroidissement des legumes. Rapport final. Eng. and Stat.Res. Centre. Agriculture Canada, Ottawa, ON. Contract Fileno.48SZ.01916-3-EP14. 58 pp.

BUDES, S. 1983. Climatisation d'immeubles au moyen deglace naturelle entreposte. Centre de Recherche Industrielledu Quebec. Ste-Foy, PQ. File no. 5-1596.197 pp.

GALLICHAND, J. and G. JACOB. 1984. Systfeme de pfcre-refroidissement des 16gumes. Rapport intfirimaire. Eng. andStat. Res. Centre. Agriculture Canada, Ottawa, ON. ContractFileno.48SZ.01916-3-EP14. 183 pp.

KADER, A.A., R.F. KASMIRE, F.G. MITCHELL, M.S.REID, N.F. SOMMER and J.F. THOMPSON. 1985. Postharvest technology of horticultural crops. Coop. Ext. Univ. ofCalifornia. Division of Agriculture and Natural Resources,Univ. of California, Davis, CA. Report no. 3311.192 pp.

VIGNEAULT, C. and M.P. LEVESQUE. 1984. Production,storage and utilization of ice for agricultural applications.Eng. and Stat. Res. Centre. Agriculture Canada, Ottawa, ON.Report no. 1-596. 15 pp.

VIGNEAULT, C. and M. LEMIEUX. 1985. Ice productionusing ambient winter temperatures. Proceedings of the ThirdInternational Conference on Energy Storage for BuildingHeating and Cooling. Public Works Canada, Ottawa, ON. pp341-344.

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